Why freeze-thaw cycles generate a gradient

Wolfgang Schechinger Wolfgang.Schechinger at med.uni-tuebingen.de
Fri Sep 8 09:47:46 EST 2000

Probably everyone already has observed this phenomenon when thawing 
coloured viscous solutions like serum. Maybe even it works with 
softdrinks, so everyone may try it at home.
Actually, it should since e.g. coke is a sugar solution.


> From:          rosie.redfield at zoology.oxford.ac.uk (Rosie Redfield)
> Subject:       Why freeze-thaw cycles generate a gradient
> Date:          8 Sep 2000 15:36:56 +0100
> Organization:  BIOSCI/MRC Human Genome Mapping Project Resource Centre
> X-To:          methods at net.bio.net
> To:            methods at hgmp.mrc.ac.uk

> The presence of a solute such as sucrose interferes with formation
> of ice crystals and so lowers the freezing point of the solution. 
> But solutions usually become less homogeneous as they freeze, and
> that's why melting causes a gradient to form.
> When a concentrated sucrose solution freezes, the tiny ice crystals
> that form first don't contain any sucrose, so the not-yet-frozen
> part of the solution is enriched in sucrose.  Eventually all the
> sucrose is trapped in between ice crystals and the solution is
> solid.
> When it slowly thaws, the first parts to melt are the parts that
> froze last, because they have the highest sucrose concentration and
> thus the lowest melting point.  As they thaw, the liquid that forms
> falls to the bottom of the tube because its high sucrose
> concentration makes it more dense than the low-sucrose ice that
> hasn't thawed yet.  The last parts to melt are the crystals that
> formed first, which contain no sucrose at all. This pure water stays
> at the top because it's lightest.
> So you can get a sucrose gradient in a single freeze-thaw step. 
> Several cycles are probably needed to give a very steep gradient.
> The formation of gradients is also why stock solutions that are
> stored frozen must be vortexed before being used.
> Rosie Redfield
> ---
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Dr. Wolfgang Schechinger, Dept. of Pathobiochemistry
University of Tuebingen, Germany
email: wolfgang.schechinger at med.uni-tuebingen.de 
wwWait: http://www.medizin.uni-tuebingen.de/~wgschech/start.htm
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